What this tool does
BMR is the calories your body would burn at complete rest — no walking, training, or digesting a big meal. This calculator applies Mifflin-St Jeor (1990) as the headline number and prints revised Harris-Benedict (Roza & Shizgal, 1984) on the same height, weight, age, and sex so you can see both equations side by side.
How it works
Enter sex, age, height, and weight (metric or imperial). The tool converts to kilograms and centimeters, plugs them into Mifflin-St Jeor, and labels the result on the card. Harris-Benedict runs on the same inputs as a sanity check. Activity only builds the comparison TDEE row — BMR itself ignores movement.
Formula and methodology
Mifflin-St Jeor (1990) — men: BMR = 10 × weight(kg) + 6.25 × height(cm) − 5 × age + 5. Women: same through age, then − 161 instead of + 5. Worked: 80 kg, 180 cm, 30-year-old man → 800 + 1,125 − 150 + 5 = 1,780 kcal/day. Revised Harris-Benedict (1984) uses different coefficients and typically lands ~50–150 kcal away — normal disagreement, not proof one equation is wrong.
How to interpret the result
If Mifflin-St Jeor BMR is 1,780 kcal, eating 1,780 kcal on a day you commute and train is a deficit. Multiply BMR by an activity factor on the TDEE calculator for maintenance. A wearable resting figure that differs by 100–200 kcal usually reflects a different definition, not a broken metabolism.
Example
A 65 kg, 165 cm, 30-year-old woman: (10×65) + (6.25×165) − (5×30) − 161 = 650 + 1,031 − 150 − 161 ≈ 1,370 kcal BMR. That is lying-still math. At moderate activity her TDEE is near 2,120 kcal. Eating 1,370 kcal daily because that is your BMR is how people accidentally under-fuel.
Common mistakes
- Treating BMR as maintenance calories.
- Picking Harris-Benedict over Mifflin-St Jeor because the number is higher.
- Ignoring that the equation only sees total weight, not muscle versus fat split.
- Comparing your BMR to someone taller without noticing size drives the math.
When this estimate may be off
- Very high muscle mass or very high body fat — total weight hides composition.
- Recent aggressive dieting, illness, or medications that shift resting expenditure.
- Age or size outside the adult population the 1990 fit was built on.
- You need measured RMR from indirect calorimetry — this page estimates only.